1. Field of the Invention
This invention relates generally to measuring formation porosity, and, more specifically, to compensating a porosity measurement obtained in a cased hole for environmental effects, including cement thickness.
2. Description of the Related Art
Innovations in the science and the art of determining the characteristics of underground geological formations have produced many advanced methodologies for the study of hydrocarbon formations. Generally, well-logging tools are passed through boreholes that are surrounded by the geological formation of interest. A source located on the well-logging tool would then irradiate the formation. Sensors strategically spaced from the source are arranged on the well-logging tool. The sensors on the well-logging tool then detect the radiation intensity or the radiation decay rate that are generally indicative of the characteristics of the geological formation that was subjected to the radiation.
Currently, the state-of-the-art method for studying geological formations, formation porosity in particular, which surround a cased borehole, is the compensated thermal neutron tool (CNL) method. Generally, the CNL tool consists of a continuous neutron source and two neutron detectors. The neutron tool bombards the cased borehole and its surrounding formation with neutrons. The sensors on the CNL tool, located at two pre-selected spacing distances from the source, then detect the neutrons. The measurement data acquired by the sensors are then processed to study the porosity of the geological formation surrounding the cased borehole. The sensors primarily measure thermal neutrons. Due to the interaction between hydrogen and neutron, the neutrons that are captured provide some indication of the porosity of the surrounding geological formation.
One of the problems in employing CNL tools in cased boreholes relates to the fact that CNL tools detect thermal neutrons. There are multiple factors in cased boreholes that facilitate thermal neutron absorption. One such factor is chlorine in the saltwater that is used in cementing the casing. The chlorine in the saltwater can function as a thermal neutron absorber. Another such factor is the absorption of thermal neutrons by the steel casing in cased wells. The absorption of the thermal neutrons tends to dilute the accuracy of the data derived from the source measurements, which can cause the formation porosity measurement to become more qualitative.
Another problem with employing the CNL tool in cased boreholes is the fact that, in the steps relating to the processing of data acquired, the cement annulus of the cased borehole is assumed to be a known factor. In many cases, the cement thickness in cased boreholes is not accurately known. This is particularly true since a certain amount of cement is simply poured around the casing of the borehole. Many assumptions regarding the thickness of the cement annulus are not accurate. Therefore, the formation porosity data that is computed using a CNL tool can be compromised due to the inaccuracies of the estimation of the cement thickness of the cased borehole.
Another well-logging tool, the Accelerator Porosity Sonde (APS), is a state-of-the-art tool that is utilized in open boreholes. The APS uses an electronic accelerator generator instead of a continuous neutron source. The APS tool generally consists of a neutron source, a neutron sensor that is positioned at a near proximity from the neutron source, a set of array sensors positioned at an intermediate proximity from the neutron source, and a neutron sensor positioned at a far proximity from the neutron source. The APS provides three different porosity measurements and a formation sigma measurement. The APS comprises a neutron source that is capable of generating pulsed neutron outputs, which allows sigma and neutron slowing-down time measurements to be made. Due to the higher neutron yield that is made possible by the APS tool, epithermal neutron measurements become more viable.
The use of epithermal neutron detection substantially reduces the effects of possible thermal neutron absorbers in the formation, providing more accurate data regarding the formation porosity. Furthermore, application of APS in an open borehole generally includes a tool standoff. Due to the relatively shallow depth of investigation that corresponds to the slowing-down time analysis, tool standoff greatly affects the measurements. The tool standoff effects must be taken into account when the formation porosity is computed. A combination of the neutron count rate ratios and the slowing-down time measurements are utilized to reduce the effects of tool standoff. Although an APS tool application has a number of advantages over a CNL tool application, the APS is currently only adapted for open borehole analysis.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.